Wireless Local Area Network (WLAN) equipment, such as clients and Access Points (APs), usually operate in an environment where the APs are fixed in location, but the client devices move about from place to place. Movement of a client device in this fashion causes changes in the performance of the link between the device and its associated AP. A client that is more distant from the AP may suffer from a loss in bandwidth and an increase in latency due to the decreasing signal strength. It is therefore of interest to quantify the performance of the WLAN devices in situations where they are mobile.
Further, the packetized data traffic that is transferred to or from such equipment over the Radio Frequency (RF) link is occasionally corrupted by errors. Such errors are commonly caused by random noise. In a situation with a high ambient noise level, for example, a very large proportion of the data transferred may be affected by such errors. It is therefore of interest when quantifying the performance of such equipment to determine how well it can function in the presence of frame errors.
Further, the incidence of errors may in fact change the functioning and internal state of WLAN equipment. For instance, an increasing level of frame errors may signal to a mobile WLAN client that it is moving away from the AP to which it is connected; basically, the increasing distance causes a decrease in the received signal strength, which in turn reduces the Signal to Noise Ratio (SNR) and thus increases the error rate. At a pre-set threshold of error rate, the client may elect to disconnect from the AP and seek out a new, closer AP to which it can connect, in the hope of decreasing its error rate. This process is referred to as roaming. It is therefore of interest to determine the behavior and performance of WLAN devices while they roam.
Additionally, the capacity of the physical link between a client and an AP is affected by the error ratio. The physical link is established by two radio transceivers, one in the client and one in the AP. The radio transceivers in WLAN equipment are typically capable of using different modulation schemes having different intrinsic bit rates and different SNR tolerances. If a client or AP finds that the modulation scheme it is currently using is leading to an excessive rate of frame errors, it may elect to drop down to a lower-rate modulation scheme that has a higher SNR tolerance in order to try to reduce the frame error ratio. This process is referred to as rate adaptation. Obviously, the use of a modulation scheme with a lower intrinsic bit rate will lead to a drop in data transfer performance between the client and AP; it is therefore of interest to quantify this performance.
In a physical sense, all of these relate to the need for WLAN equipment to operate with a spatial distance between them. If the equipment were statically located in close proximity, they would receive signals with a high signal strength (relative to the ambient noise level) and would therefore suffer a low rate of errors. Also, there would be no need for a client to roam to a new AP. However, the prime characteristic of WLAN equipment being the support of mobility, it is necessary for them to operate at substantial distances from each other. Increasing distance leads to an attenuation of the RF signal, a reduction in received signal strength, and a consequent increase in frame errors.
Two principal metrics have been used to quantify the mobility performance of WLAN equipment. The first is commonly known as the rate versus range performance metric, and consists of measuring the data transfer performance as a function of distance. The second is known as the roaming performance metric, and consists of observing the behavior and performance of a WLAN system when a client roams from one AP to another; roaming occurs when the RF signal level and the incidence of frame errors rises above some threshold.
Heretofore, the measurement of the mobility performance of WLAN equipment has been performed by introducing an actual attenuation of the RF signal between two pieces of WLAN equipment to induce the effects described above, and then measuring performance according to the desired metric. The approaches that have been implemented to date to perform such measurements include:                (a) Placing the WLAN equipment (both real clients and real APs) within a large physical space, and then physically moving the devices in order to increase or decrease the distance between them. Thus the rate versus range metric can be measured by measuring the data transfer rate of the WLAN equipment at different physical distances from each other, and the roaming metric can be obtained by observing the behavior of the WLAN clients and APs as the relative distances between them change. Unfortunately, this method requires a large floor space, is highly labor-intensive, subject to variations due to human error, and is prone to interference and external signals. It is thus neither cost-effective nor repeatable. It is also very difficult to predict or control the level of signal and the amount of frame errors that will be induced during the test. It is thus unsuitable for tests involving controllable frame error ratios.        (b) Placing the WLAN equipment (again, real clients and Teal APs) into separate shielded chambers that are interconnected by means of variable RF attenuators. Increasing the amount of attenuation simulates the increase of distance between the WLAN equipment; conversely, decreasing the amount of attenuation simulates a decreasing distance. This approach avoids most of the problems of the first mentioned method; it can be automated and is relatively efficient. Further, it affords some degree of control of the frame errors and signal levels. However, it suffers from unpredictable variations due to the manufacturing tolerances of the client and AP radio transceivers. Also, it relies on the use of both clients and APs in the same test; as the characteristics of both such equipment are widely variable, this method forces the user to test every combination of client and AP in order to quantify just the performance of, for instance, the AP. Finally, the level of frame errors that are produced cannot be easily predicted, but must be experimentally measured. It would be much preferred to replace one side of the WLAN link (i.e., either the client or the AP) with a piece of dedicated WLAN test equipment, and test the other side of the WLAN link (i.e., the AP or client, respectively) without being subject to manufacturing tolerances.        
Accordingly, there is a need for improved systems and methods for enabling the efficient measurement of the mobility performance of WLAN equipment. Further, there is a need for methods for controllably simulating distance between a WLAN device and a WLAN tester without being subject to RF noise or manufacturing tolerances. Further, there is a need for systems and methods for controlling the effective frame error ratio introduced into a WLAN link (in either receive or transmit directions, or both) without the use of RF attenuators.